The present invention relates to input/output (IO) cells in general and to high speed IO cells using an auxiliary power supply in particular.
A goal in integrated circuit manufacturing is to increase circuit density and functionality. Accordingly, there has been a great deal of effort put into reducing the size of individual transistors so that more transistors, and thus more functionality, can be placed on each device.
There is a downside to these higher densities and smaller devices. For example, smaller devices can only standoff or support a limited voltage before breakdown occurs. Higher densities can result in an increase in power supply dissipation per unit area of an integrated circuit, which can limit operability and lifetime, as measure in mean time before failure, of the circuit. To mitigate both these consequences, the power supply voltages applied to integrated circuits has been progressively lowered over the years, from 5 volts to 3.3, then to 2.5 and recently 1.8 volts and even lower.
This reduction in power supply voltage has taken its toll on some of the individual circuits that are used in the design and manufacture of integrated circuits. One type of circuit that has been particularly effected are output drivers. The reduction in supply voltage has meant a corresponding decrease in their drive capability.
To compensate for this, the size of output devices has often been increased. This has the undesirable results of consuming more die area for the output drivers, and also necessitate the increase in size of predriver circuits that drive the output drivers themselves. The increase in size of these circuits increases their power supply currents, thus offsetting some of the gains achieved by having the smaller devices and lower power supplies in the first place. Alternately, a slower output driver may be used, but these outputs are more susceptible to noise and jitter.
Accordingly, what is needed are circuits, methods, and apparatus that provide fast output drivers using lower power supplies but do not require large integrated circuit areas for their implementation.